This is a great question. Why is Missoula called "Zootown"? Well according to the Urban Dictionary there are a couple reasons.

People have their own thoughts on why it's called that but according to Urban Dictionary...

Zootown : A nickname for the small city of Missoula in the mountains of western Montana. Missoula became known as Zootown for two reasons first it sounds like the word zoo, Mostly however it is because Missoula is known for its wild parties and people, it is home to the U of M, and on any given night you never have to walk more than two blocks to find a wild party. Zootown partys are wild.

I'm pretty sure we can debate that not everyone in this town is wild and not all parties are out of control. Why do you think it's called that? Is it maybe because of the big amount of wildlife, like a Zoo?